Synthetic oils have been used in aircraft turbine engines for over 40 years, engine manufacturers approve certain oils which can be used in their engines, the operator makes the choice. If it is TCP which is causing the problem there may not be any alternatives. I would have thought that if enough MOR's in the UK are raised concerning possible fumes in the cockpit the CAA would have to carry out a proper investigation. The obvious solution is for cabin air to be sourced externally and not from the engine compressor.